A conventional diaphragm spring-type clutch assembly for an automotive vehicle includes an engine driven flywheel, a clutch cover secured to the flywheel, an axially reciprocable pressure plate rotatable with the cover and flywheel, a clutch driven plate assembly mounted on the transmission input shaft extending into the clutch cover and having friction surfaces located between engaging surfaces of the flywheel and pressure plate, a diaphragm spring pivotally mounted on the clutch cover and engaging a pressure plate fulcrum surface adjacent the spring periphery, and an axially reciprocable release bearing movable on the transmission input shaft and engaging the inner ends of spring fingers formed on the inner portion of the diaphragm spring.
Inward movement of the release bearing due to the vehicle operator depressing the clutch pedal causes the bearing to engage the spring fingers and pivot the diaphragm spring about the privotal mounting means secured in the clutch cover to release the pressure plate. Release of the clutch pedal causes the diaphragm spring to pivot in the opposite direction to engage the pressure plate and urge it against the clutch plate, thus squeezing the clutch plate between the flywheel and pressure plate to engage the clutch. As the clutch is engaged and disengaged numerous times during operation of the vehicle, the diaphragm spring is stressed by the cyclical pivotal movement thereof during each clutch engagement and disengagement. Work has been performed studying the cyclical stress encountered by the diaphragm spring and efforts have been made to reduce the maximum stress.
Photoelastic tests performed on a diaphragm spring show that the tensile and compressive stress is extremely high at and beyond the valley load of a typical deflection rate curve for a diaphragm or Belleville spring. These high stresses can cause a reduction of the effective life of the spring, and it was found that a small reduction in maximum cyclic stress can result in large gains in the spring fatigue life. The present invention provides an effective way of reducing the maximum cyclic stress of a diaphragm spring in a vehicle clutch.